Rafael lives near a road at the bottom of the hill. His parents are concerned that soil will wash off the hill and rocks will fall onto the road. Rafeal converts an investigation to find out if grass growing on a hillside will help stop soil erosion. He collects two samples of the same size and type of soil. One sample of soil has grass growing on it and the other does not. He places each sample of soil in a small tray. For his investigation, Rafael pours the same amount of water onto both samples of soil. He uses a large tray to collect water that may flow through the soil. Which practice shows the best way for Rafael to set up this investigation?

The experimental set-up must be in a sloppy position in order to simulate the condition that is characteristic of the bottom of the hill where he stays. Hence, the two soil samples must be tilted in order to determine if the grass can help to stop the washing down of the soil from the top of the hill.

The two options following the first option do not fulfill the condition of the same treatment. Both soil samples must be subjected to the same treatment condition in order for the outcome to be valid. The last option could have also been admissible but did not simulate the hill/slope conditions of where Rafael stays.